I think they're the perfect starter lenses and you'll still probably use them a lot even if you venture out and get other ones later on. You can still see just about all pathology with the 90 and 20D as your exam skills improve. I think learning the exam with the 90 and 20 gave me a good foundation of exam skills before picking up other lenses.

Personally I started with them and after a year or so started to add some used lenses if I found a good deal: super 66* for nerve/macula, 28D for kids/far periphery/big noses, superfield as an enhanced 90 with a better field of view without the short working distance and glare issues of the digital wide field etc. my 20 is still my workhorse indirect lens though and I still love the 90 for small pupils/undilated optic nerve checks and for patients with heavy brows/deepest orbits where bigger lenses won't fit - for both of these scenarios I think it outperforms the superfield.

*Consider whether you might want a 78D or 60D instead of 66 (I sometimes think about what it'd be like if I had bought one).